Gas Metal Arc ("GMA") welding is a welding process that produces coalescence of metals by heating them with an arc current passed across a gap between a consumable filler metal electrode and a workpiece. Gas shielding is provided by an externally supplied inert gas or gas mixture to prevent contamination of the coalescing metals during welding.
Pulsed GMA welding is one form of GMA welding which uses a pulsed arc current waveform having a background arc current component and a pulse arc current component. The background current maintains a minimum arc current and preheats and conditions the electrode wire for subsequent melting. The background current must not be so high as to cause arc burnback. The pulse current component, having a larger amplitude than the background current, heats the electrode to a temperature above its melting point, causes spray metal transfer to occur, and increases the distance of the gap. Spray metal transfer involves the movement of molten filler metal of the consumable electrode, under electrostatic forces, from the end of the electrode wire in an axial stream of fine droplets across the air gap between the end of the electrode and the work piece. The droplets typically have a diameter less than that of the electrode wire. Pused GMA welding typically uses an argon or argon-rich mixture to envelope the electrode and workpiece about the air gap.
As spray transfer occurs, the end of the electrode wire nearest the workpiece retreats from the workpiece and increases the air gap or arc length. Because the electrode wire is typically advanced at either a relatively constant rate or at a pulsed rate, the arc length can vary within a given range by careful selection of four welding current characteristics--amplitude and duration of the background current, and the amplitude and duration of the pulse current--and a wire feed rate so that the filler metal of the electrode wire will be melted or burned off and transferred to the work piece at a rate that will keep the end of the electrode wire at or within a given distance from the work piece. The melting must occur within the desired arc length range to provide acceptable spray metal transfer. If melting occurs outside the desired range, the metal transfer may be globular or a short circuit drop deposit, or no metal transfer will occur at all, resulting in irregular and unaccaptable weld deposits on the work piece. The burn-off is accomplished by changes in the arc current intensity as the wire moves and the arc length and arc voltage change. GMA welding has been previously defined in Metals Handbook, Ninth Edition, Volume 6 "Welding, Brazing and Soldering", pages 153-181, American Society For Metals (1983).
Conventional GMA welding machines comprise a power supply sufficient to generate an arc current, a wire feeder to advance the electrode wire as it is melted, a flow of shielding gas, and a welding device or gun that carries the current, electrode wire, shielding gas, and, if necessary, cooling water. Conventional welding guns and shielding gas flows are known to those skilled in the art and do not form a part of this invention.
Power supplies used in arc welding machines are desired to be autoregulating so as to provide as much current as is needed to maintain a stable arc length and spray metal transfer as the electrode wire advances and the instantaneous arc length changes. The current characteristics for the particular electrode wire must take into account the size, mass, and composition of the filler metal electrode and must be selected so that the wire will be alternatively preheated and then melted. The selection of the electrode wire and the welding conditions must also take into amount the mass and composition of the workpiece, and the nature of the desired weld.
The arc voltage is the voltage drop across the arc length or air gap, and its magnitude is related to the size of the arc length. The relationship between the output voltage of the power supply and the output current delivered to the electrode, the arc current, forms the output characteristic of the power supply. The output characteristic determines what the corresponding voltages and currents will be for all the operating points of the power supply or conditions from when the electrode contacts the workpiece to when the arc length is large so that substantially no arc current flows. The amplitude and duration of the pulse current determine how fast the wire is melted away and the size of the droplets. The rate a which the wire is melted must be at or exceed the rate of wire feed to maintain a stable arc length. As the arc length changes, the power supply must be able to respond to the changing arc voltage with arc currents necessary to preheat or melt the wire and thereby to maintain the arc length in the stable range.
One known GMA welding machine uses two power supplies. The first power supply provides the background current and the second supply providing the pulse current. The second power supply may be half or full wave rectified current which may be tied into a line current to provide spray transfer at a rate of 60 or 120 times per second. The problem with this type of device is that it requires two power supplies adding to the power consumption, weight, and cost of the apparatus without easy variance of pulse current duration to control droplet size.
Another known GMA welding machine uses a single power supply that has an output characteristic which may be flat, slighty uprising or slightly drooping. This output characteristic is designed to provide large or rapid changes in current over a relatively small arc length or arc voltage change. Other known power supplies have nonlinear output characteristics including a background current segment and a pulse current segment with a transient current segment for shifting between background and pulse currents as the arc length changes. Such nonlinear characteristics may have a steeply drooping outpul characteristic for maintaining a stable background current and preheating the electrode and a flat output characteristic for the pulse current.
The problem with these systems is that the power supplies can neither adequately control the changes in arc current from background to pulse and back to be autoregulating nor provide the stable arc current amplitudes and durations needed to preheat and then melt the electrode wire. Consequently a stable spray metal transfer is not achieved over sustained use. Instability can result from the electrode wire contacting the work piece, the arc self-extinguishing or burning back, or the electrode wire being melted back to a point too far from the work piece to remain within the autoregulating capability of the welding machine.
One welding machine power supply that attempts to control the output current level is shown in Amin, M., Metal Construction, page 349, June 1981. An electronic control system adjusts the pulse current repeat frequency, amplitude, duration, and background arc current in response to the wire feed rate and automatically alters the current parameters to balance the wire feed and wire burn-off rates, to establish a constant arc length. The problem with this system is that there is no direct relationship between the arc length and the four welding parameters so that any change in arc length, not a result of the wire feed rate, will destroy the arc burning stability.
Another known power supply, as shown in Ueguri European Patent Application No. 81105288.5, controls the power source output by responding to the detected arc voltage. An integrated circuit is used to detect the arc length and corresponding arc voltage for appropriately adjusting the four welding parameters during the welding process. The problem with this approach is that the transient response of the integrated circuit control apparatus is not fast enough to correct for disturbances as disturbances occur, defeating the autoregulating ability.
Several other problems are associated with conventional methods of signal detecting and conventional control circuits for electronic switching. For example, due to static error of closed loop feedback and the variation of position and slope of segments, it is very difficult to superimpose the arc current transfer point and the real crossover point of nonlinear output characteristic segments. The error resulting from attempting to connect the transfer point and the real crossover point creates a dead zone, an overlap zone, or an oscillation zone near the crossover point. This mismatch of the multi-segmental output characteristic results in inadequate autoregulating performance.
It is therefore an object of this invention to provide an autoregulating power supply that responds to changing arc voltage and arc length by providing a proper arc current to maintain stable GMA welding.
Another object of this invention is to provide a power supply having an output characteristic what is muti-segmented without significant crossover mismatch and has a very fast response time to maintain stable GMA weding.
It is another object of this invention to provide a pulsed GMA welding power supply that has a continuous, nonlinear moving output characteristic which is dependent upon the arc voltage and is monitored by a scanning circuit to provide the arc current required in an amplitude and for a duration appropriate for stable arc and spray metal transfer.